208 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



apparatus can be employed. The interocular distance is varied by turn- 

 ing the milled ring (C, fig. 22) on the direct tube of the Microscope ; 

 this causes both draw-tubes to move in or out, and alters the distance 

 between the oculars from 2 in. to 2J in., which, as the observer's eyes 

 cannot be in contact with the eye-pieces, represents interocular distances 

 of about 2{ in. to 2f in. The tube-length is the standard 160 mm. at 

 an intermediate position. For those whose eyes are farther apart than 

 this, special tubes can be constructed to give the extra separation. If the 

 two eyes of an observer are dissimilar, the necessary lens to render them 

 equal is supplied in a cap to fit over the eye-piece. 



Doubt has been at times expressed as to whether a Microscope look- 

 ing at an object with a single object-glass can under any circum- 

 stances give a really stereoscopic relief. Those who have worked with 

 a binocular Microscope do not retain such a doubt, and the explanation 

 of the phenomenon is quite satisfactory. Suppose that 0, fig. 25, re- 

 presents the objective, and that an object at X consists of a fine blade 



A B 



Fig. 24. 



Fig. 26. 



of material placed on end, all the light from the left hand of this blade 

 which enters the object-glass at nil reaches the left hand of the lens only, 

 and from the right-hand side of X reaches the right-hand side only. If 

 the light from the lens is geometrically divided and passed to one eye 

 at A, and the other at B, a perfect stereoscopic picture will result, as 

 though the eyes were looking on both sides of a card held in front of 

 them in the well-known experiment on binocular vision. A Microscope 

 inverts the image, and consequently to pass the correct image to the eyes 

 to obtain the stereoscopic relief, the light from the right-hand side of the 

 object-glass must be passed to the left eye, and vice versa, By examin- 

 ing the diagram of the rays passing through a Microscope, as indicated in 

 fig. 23, it will lie seen that the rays of light intermingle after they leave 

 the object-glass, and at no other place between the lenses could the 

 right-hand half of the rays entering the object-glass be separated from 

 the left half. It will, however, be noticed in fig. 23 that all the rays of 

 light, after passing through the Microscope, pass through the Ramsden 

 circle (ZZ 1 ) just above the eye-piece. 



The observer naturally places his eyes so that the whole of the 

 Ramsden disks (fig. 26) enter the pupils of the eyes, and he thus obtains 

 all the advantages, as to aperture, resolution, and illumination, of a 



